Whitefly-transmitted geminiviruses have become a major limiting factor in tomato production in Florida, the Caribbean and much of Latin America. This group of viruses is currently expanding in the Western Hemisphere, and the number of characterized geminiviruses which infect tomato in this region has increased from three to more than 17 over the last 15 years (Polston and Anderson, 1997). This expansion is continuing and reports of new epidemics are appearing almost monthly. Whitefly-transmitted viruses appear alone and in mixed infections with other geminiviruses and other viruses. Whitefly-transmitted geminiviruses are reducing tomato yields in many countries, and total crop losses are not uncommon (Polston and Anderson, 1997). In Florida, tomato production has suffered significant losses (estimated at $125 million in 1990-91) due to Tomato mottle virus (ToMoV) infection, which first appeared in 1989 and to Tomato yellow leaf curl virus (TYLCV), which has caused crop failures and increases in production costs due to increases in pesticide use.
The family Geminiviridae is divided into three genera, of which the genus, Begomovirus, contains the whitefly-transmitted geminiviruses. There are two major divisions within the genus Begomovirus, those with monopartite genomes and those with bipartite genomes. The taxonomy of Begomoviruses is in a state of confusion, due to the naming of viruses based on disease symptoms before any sequence analysis has been performed. Many of these viruses cause similar symptoms in the same host, which has resulted in very different viruses having the same or related name in the literature. This is particularly true for tomato yellow leaf curl virus (TYLCV). There are eight separate and unique Begomoviruses named TYLCV. Recent suggestions to resolve this confusion have been published, though none have been adopted by all virologists, with the result that different viruses are referred to in a number of different ways. Using the scheme suggested by Fauquet and Mayo (1999), the eight viruses are identified based on the country from where the virus was first described: from China (TYLCV-Ch), from Israel (also the first one to be described) (TYLCV-Is), from Nigeria (TYLCV-Ng), from Sardinia (TYLCV-Sar), from southern Saudi Arabia (TYLCV-SSA), from Tanzania (TYLCV-Tz), from Thailand (TYLCV-Th), and from Yemen (TYLCV-Ye). Thus, while each of these viruses is named TYLCV, each of these viruses have unique sequences with less than 90% sequence homology over the genome.
The geminivirus referred to as tomato yellow leaf curl virus-Israel (TYLCV-Is), which caused extensive losses to tomato production in the Dominican Republic (reviewed by Polston and Anderson, 1997), was found in Florida in 1997 (Polston et al., 1999). TYLCV-Is infection of tomato is a serious problem in the United States Florida, Georgia, Louisiana, and Mississippi), the Caribbean (The Bahamas, Cuba, Dominican Republic, Puerto Rico, and Jamaica), Mexico, Japan, Europe, and the Mediterranean (Canary Islands, Egypt, Israel, Cyprus, Italy, Spain, Portugal and Morocco). Incidences of TYLCV-Is are increasing and economic losses have been experienced as recently as fall of 1998. TYLCV-Is virus is widespread in Florida, is likely to increase over the next few years and will become a major constraint to tomato production in Florida. Currently, there are few commercial tomato cultivars on the market with resistance to infection by TYLCV-Is. The resistance is categorized as tolerance, since infected plants show no to slight symptoms and produce yields that are relatively unaffected by infection; however, the virus can still be detected in inoculated plants and these plants can serve as sources of inoculum for susceptible cultivars and crops (Lapidot et al., 2001).
Geminiviruses are very difficult to economically manage in fresh market tomatoes, and practically impossible to manage in processing tomatoes. At this time, geminiviruses are managed primarily through the use of a single insecticide, imidacloprid (Bayer Agricultural Products, Kansas City, Mo.), to reduce the population of the whitefly vector. Tolerance to this insecticide has already been reported (Cahill et al., 1996; Williams et al., 1996). It may be only a matter of time before imidacloprid loses efficacy in the United States and other locations. The average Florida tomato grower spent approximately $250/acre for insecticides to control ToMoV in 1994 through 1997. These costs have increased significantly as U.S. growers' struggle to manage TYLCV-Is. In Caribbean countries geminiviruses have caused many small and medium size tomato growers to go out of business due the increases in costs of production and crop losses. In Israel, where imidacloprid resistance is present, TYLCV-Is is managed by pesticide use plus exclusion; tomatoes are produced in planthouses enclosed in whitefly-proof screening material or in screened tunnels in the field. The use of these methods is expensive and is often not an economically or horticulturally realistic alternative. The least expensive and most practical control of whitefly-transmitted geminiviruses is the use of resistant cultivars. At this time there are no commercially available resistant tomato cultivars for the geminiviruses native to the Western Hemisphere. As noted above, there are only two cultivars with tolerance to TYLCV-Is that are suitable for production in the U.S. This resistance is derived from wild species of Lycopersicon, probably L. peruvianum and L. pimpinellifolium. 
There are a few reports suggesting that the gene encoding the Begomovirus replication associated protein (Rep) might be used for resistance to viral infection. There has been a report that a modified ToMoV Rep mutated in an NTP-binding motif was transformed into tomato plants and demonstrated to interfere with viral replication (Stout et al., 1997). Hanson et al. (1999) analyzed phenotypes of BGYMV (bean golden yellow mosaic virus) with mutations in an NTP-binding motif of the Rep gene, and demonstrated that the NTP-binding domain is required for replication. They proposed that mutations in this motif might serve in a trans-dominant negative interference scheme for pathogen-derived resistance (also known as “dominant negative mutations”). Resistance to African cassava mosaic geminivirus (ACMV) in Nicotiana benthamiana plants was developed by transformation with ACMV Rep (Hong and Stanley, 1996).
Some viral resistance has been reported using the Rep gene of the geminivirus TYLCV from Sardinia (TYLCV-Sar). However, TYLCV-Sar is a distinct virus, only distantly related to TYLCV-Is and shows significant differences in the genomic sequence (<80% homology). Noris et al (1996) found TYLCV-Sar resistance in N. benthamiana plants using the TYLCV-Sar Cl gene that encodes a protein with a truncated C-terminal (210 amino acids). However, resistance was overcome with time. Brunetti et al. (1997) transformed tomatoes with the same construct and found that high accumulation of the truncated Rep protein was required for resistance, that high accumulation resulted in a “curled” phenotype, and that the resistance did not extend to an unrelated geminivirus. Transgenic Nicotiana benthamiana plants expressing antisense RNA to the Rep gene have been observed with resistance to TYLCV-Sar infection (Bendahmane and Gronenborn, 1997). The plants transformed according to the methods of the subject invention have a normal phenotype and are expected to be high yielding as well.
The present invention overcomes the expensive, time consuming, and limited options in the traditional plant breeding programs applied to generating virus resistant tomato cultivars.